It is well known that color photographic light-sensitive elements, using the subtractive process for color reproduction, comprise silver halide emulsion layers selectively sensitive to blue, green and red light and associated with yellow, magenta and cyan dye-forming couplers which form (upon reaction with an oxidized primary amine type color developing agent) the complementary color thereof. For example, an acylacetanilide type coupler is used to form a yellow color image; a 5-pyrazolone, pyrazolotriazole, cyanacetophenone or indazolone type coupler is used to form a magenta color image; and a phenol type, such as a phenol or naphthol, coupler is used to form a cyan color image.
Usually, the color photographic light-sensitive elements comprise non-diffusible couplers incorporated independently in each of the light-sensitive layers of the material (incorporated coupler materials). Therefore, a color photographic light-sensitive element usually comprises 1) a blue-sensitive silver halide emulsion layer (or layers) which contains a yellow dye-forming coupler and which is mainly sensitive to blue light (substantially to wavelengths less than about 500 nm); 2) a green-sensitive silver halide emulsion layer (or layers) which contains a magenta dye-forming coupler and which is mainly sensitive to green light (substantially to wavelengths of about 500 to 600 nm); and 3) a red-sensitive silver halide emulsion layer (or layers) which contains a cyan dye-forming coupler and which is mainly sensitive to red light (substantially to wavelengths longer than about 590 nm).
It is also known that 5-pyrazolones in which the 4-position of the pyrazolone ring is free, that is having only hydrogen substituents (4-equivalent magenta couplers), can be used as magenta couplers in color photographic elements to provide magenta dye images having useful properties. Examples of such couplers are the 4-equivalent 3-anilino-5-pyrazolone couplers described in, for example, U.S. Pat. Nos. 3,519,429, 3,907,571, 3,928,044, 3,935,015 and 4,199,361. However, 4-equivalent 5-pyrazolone couplers have a number of disadvantages, as they require four equivalents of silver to produce each molecule of dye, are sensitive to certain chemical vapors, for example formaldehyde, and have poor dye light and dye dark stability. These drawbacks can be overcome by using so-called 2-equivalent 5-pyrazolone magenta couplers in which a substituent is introduced into the coupling position (4-position) of the coupler and eliminated as a leaving group (coupling-off group or splitting-off groups) during the color development process, thus requiring only two equivalent of silver in order to produce each molecule of dye.
Among coupling-off groups known in this connection are the arylthio groups described, for example, in U.S. Pat. Nos. 3,227,554, 3,701,783, 3,935,015, 4,351,897, 4,413,054, 4,556,630, 4,584,266, 4,740,438, 4,853,319, 4,876,182, 4,900,657, 4,929,540, 4,942,116, 5,250,407, 5,262,292, and 5,256,528; WO 88/04795, 92/18902, and 93/02393; EP 341,204, and GB 1,494,777.
The silver halide emulsions used in the past for such photographic elements were the so-called mixed emulsions, that is, emulsions comprising a combination of a more sensitive emulsion (containing coarse silver halide grains) and a less sensitive emulsion (containing fine silver halide grains) whereby a straight density-log exposure curve could be obtained for each blue-, green- and red-sensitive layer.
Since granularity of the dye image in color photographic elements depends mainly upon the size of the silver halide grains employed, attempts to increase the sensitivity of the color photographic material by increasing the size of the silver halide grains (sensitivity of silver halide grains generally is proportional to the size of the silver halide grains) caused a coarsening of the granularity of the dye image.
As a method for improving sensitivity, a technique has been known in which the regular layer sequence of having respective red-sensitive, green-sensitive and blue-sensitive silver halide emulsion layers is provided by subdividing a part or whole of each of the emulsion layers into higher and lower sensitivity emulsion layers, each subdivided layer containing a color coupler forming substantially the same hue as the other subdivided layer and wherein these layers are coated adjacent to each other.
For example, GB 818,687 describes a method for increasing sensitivity in multilayer color photographic elements in which the emulsion layer which is applied closest to the support consists of two partial layers sensitized to the same region of the spectrum, the lower layer consisting of a less sensitive silver halide emulsion layer and the upper layer consisting of a more sensitive silver halide emulsion, both partial layers containing color-forming couplers in the same concentration. An element of this type has, however, the disadvantage that the increase in sensitivity is accompanied by an increase of granularity.
To overcome this disadvantage and lower the granularity of color images, GB 923,045 describes a method for increasing the sensitivity of a color photographic element without coarsening the granularity of the dye image by providing an uppermost more sensitive emulsion layer and a lowermost less sensitive emulsion layer, both layers being sensitive to the same region of the visible spectrum and each containing non-diffusing color couplers, with the maximum color density of the more sensitive emulsion layer being adjusted to be lower than that of the less sensitive emulsion layer, in particular being lower in an amount from 0.20 to 0.60.
U.S. Pat. No. 3,516,831 describes a process for improving the sharpness of the color image, according to which two layers which are sensitized to the same spectral region of the spectrum contain different couplers, the more sensitive emulsion layer containing 4-equivalent couplers and the less sensitive emulsion layer 2-equivalent couplers.
Both processes described in GB 923,045 and U.S. Pat. No. 3,516,831 have numerous disadvantages, for example, a worsening of granularity in high-sensitivity photo-graphic elements. A process for improving granularity is described in U.S. Pat. No. 3,726,681 wherein granularity of high-sensitivity photographic elements is improved by using a coupler having a fast coupling reaction rate in a more sensitive silver halide emulsion layer and a coupler having a slow coupling reaction rate in a less sensitive silver halide emulsion layer. Since, however, sharpness is not sufficiently improved, EP 107,112 describes a color photographic element in which at least one of the silver halide emulsion layers is composed of two silver halide emulsion layers sensitive to the same color, the more sensitive layer containing a high reaction rate coupler, and the less sensitive silver halide emulsion layer containing a low reaction rate coupler in a range of 1/1.3 to 1/15 of that of the high reaction rate coupler and a diffusible DIR (Development Inhibitor Releasing) coupler. The purpose of DIR couplers is to help in reducing graininess and improve sharpness of the image due to intralayer or intraimage effects (that is in the same layers or the same dye image) and improve color reproduction due to interlayer or interimage effects (that is effect between different layers or different dye images).
Recently, the picture size of photographic films has been reduced to miniaturize photo cameras, and silver halide grains have become coarser to increase sensitivity of photographic elements. Therefore, the degrading tendency of the granularity has been increased, even if the aforesaid double silver halide emulsion layer system is used.
U.S. Pat. No. 3,843,369 describes a method for further increasing the sensitivity of a color photographic element by providing three emulsion layers sensitive to the same spectral region of visible light and comprising image forming couplers, the uppermost silver halide emulsion layer having the highest light sensitivity and the lowermost silver halide emulsion layer having the lowest light sensitivity, the uppermost and the intermediate layer each having a maximum color density of 0.6 or less obtained by increasing the stoichiometrical molar ratio of the silver halide to the coupler up to about from 20 to 120.
U.S. Pat. No. 4,145,219 describes high speed and low granularity multilayer color photographic elements comprising red-sensitive, green-sensitive and blue-sensitive silver halide emulsion layers containing image forming couplers, wherein at least one of the silver halide emulsion layers comprises an upper unit silver halide emulsion layer, a middle unit silver halide emulsion layer and a lower unit silver halide emulsion layer, with each of the three unit layers being sensitive to visible light in the same spectral wavelength range, with sensitivity of the unit layers decreasing towards the lower unit layer, and with the middle unit layer containing a DIR coupler. In the unit layers, the amount of couplers in the upper unit layer is reduced so that the molar ratio of the silver halide to the coupler is 20:1 to 150:1, by which the maximum color density of the image becomes 0.6 to 0.1, while the molar ratios of the silver halide to the coupler in the middle unit layer and that in the lower unit layer are 10:1 to 100:1 and 2:1 to 5:1, respectively.
U.S. Pat. No. 4,564,587 discloses a light-sensitive silver halide color photographic material wherein at least one light-sensitive layer is constituted of a plurality of silver halide emulsion layers having the same color sensitiveness but being different in sensitivities and containing dye image-forming couplers, wherein the plurality of layers is provided by coating in the order from the support side a low sensitivity layer, a medium sensitivity layer and a high sensitivity layer, the density of coupler in the medium sensitivity layer is 10 to 60% of the coupler density in the low sensitivity layer, and the maximum color density in the medium sensitivity layer is between 0.6 and 1.2.
U.S. Pat. No. 4,582,780 describes a method for increasing sensitivity and improving adjacency effects by providing three emulsion layers sensitive to the same spectral region of visible light, the uppermost silver halide emulsion layer having the highest light sensitivity and the lowermost silver halide emulsion layer having the lowest light sensitivity, wherein the maximum color density of the uppermost silver halide emulsion layer, after color development, is lower than 0.60 and the maximum color densities of both the intermediate and the lowermost silver halide emulsion layers, after color development, are each higher than 0.60.
EP 583,020 discloses a technique for improving granularity by providing a multilayer color photographic element comprising a plurality of blue, green and three red sensitive silver halide emulsion layers, the layers being arranged on the support in the sequence: a red least sensitive layer, a green least sensitive layer, a red mid-sensitive layer, a red most sensitive layer, a green most sensitive layer, a blue most sensitive layer, and a blue least sensitive layer.
EP 608,464 discloses a technique for enhancing the speed-granularity relationship of dye images by providing multicolor photographic elements containing blue, green and red sensitive layer units wherein at least one layer unit contains three superimposed silver halide emulsion layers of different sensitivity comprising silver bromoiodide tabular grains of different iodide content.
Generally, the multilayer color photographic elements comprising three silver halide emulsion layers sensitive to the same spectral region of the visible light of different sensitivity have in the highest sensitivity uppermost layer a weight ratio of the coupler to the silver halide (expressed as silver) which is lower than or equal to that of the medium sensitivity intermediate layer and much lower than that of the lowest sensitivity lowermost layer. As known in the art, the highest sensitivity uppermost layer is a coupler "starved" layer, that is a layer in which there is much less dye-forming coupler than is theoretically capable of reacting with all the oxidized developing agent generated at maximum exposure.
As hereinbefore described, 2-equivalent 5-pyrazolone magenta couplers having an arylthio group attached to the 4-position of the pyrazolone ring have a number of advantages compared to 4-equivalent 5-pyrazolone magenta couplers in which the 4-position of the pyrazolone ring is free (that is having only hydrogen atoms). For example, 2-equivalent 5-pyrazolone couplers require only two equivalent of silver to produce each molecule of dye, are less sensitive to certain chemical vapors, for example formaldehyde, and have high dye light and dye dark stability. However, 2-equivalent 5-pyrazolone magenta couplers have the disadvantage that they may cause worsening of granularity and interimage effects when used in all the three silver halide emulsion layers sensitive to the green spectral region of the visible spectrum.